[opendtv] Re: Europe now debates i vs p
- From: Mark Schubin <tvmark@xxxxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Mon, 13 Sep 2004 23:53:08 -0400
Given that it's possible to detect modulation from spacecraft beyond the
orbit of Pluto, it should come as no surprise that it's possible to
detect filament-current changes. But that's not what a television
camera is designed to do. It integrates light over a period that is a
substantial fraction of (or, in the case of a 25-frame camera looking at
a 60 Hz bulb, a period greater than) the filament-current period. And
the filament current drops below 50% for only 1/3 of each sinusoidal
cycle -- and that in two sub-periods per cycle. And, even then, there's
a thermal "flywheel" effect.
If the mismatching of camera frame rate and incandescent lighting power
frequency were really a problem, then it would be a problem even for
NTSC in a 60 Hz environment due to the mismatch of 60 Hz and 29.97
frames per second. It would also be a problem for motion-picture film
cameras, especially when they shoot slow motion. And a still camera
shooting at, say, 1/200 of a second would be expected to deliver wildly
different exposures under AC incandescent lighting, depending on the
part of the power cycle during which the exposure took place.
Well, out with your still cameras! Shoot! Are the exposures wildly
different? Try 1/500 of a second. Got any wildly differing exposures yet?
I have been involved in many projects where NTSC cameras were used in 50
Hz countries and 25-fps cameras were used in 60 Hz countries. In all
cases, low-frequency fluorescent and arc lights were problematic. In no
cases were incandescent lights a problem.
TTFN,
Mark
Craig Birkmaier wrote:
>At 8:58 AM +0200 9/13/04, jeroen.stessen@xxxxxxxxxxx wrote:
>
>
>>I'd say that Doug is right. I remember long time ago a hobby
>>project (in a magazine) where a lamp was used to carry speech
>>over a distance, by modulating the filament current. While it
>>may take a long time for a filament to cool off completely,
>>initially the cooling goes very fast. It has to do with the
>>radiated power being proportional to the FOURTH power of the
>>temperature. Thin white hot metal will cool very quickly !
>>
>>
>
>YUP.
>
>We are talking about the ability to perceive the modulation in
>intensity of the "flickering" light source.
>
>Regards
>Craig
>
>
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